Audiometric signal and apparatus for producing such signal

ABSTRACT

An audiometric signal for testing hearing is disclosed having alternating first and second portions, the first portion having a known frequency and a first amplitude, the second portion having the same known frequency and a second amplitude, the change of amplitude occurring in response to the zero crossing of a duty cycle base signal and coinciding with a zero crossing of the audiometric signal. In one exemplar embodiment, apparatus for generating the audiometric signal is provided, comprising means for generating a sinusoidal signal of preselected audio frequency, means for generating a duty cycle base signal of preselected frequency, gating means for receiving the duty cycle base and sinusoidal audio signals and alternately passing the sinusoidal audio signal of preselected frequency at first and second differing amplitudes, the change in amplitude occurring in response to the zero crossing of the duty cycle base signal and coinciding with a zero crossing of the sinusoidal audio signal, amplifying means for amplifying the signal, and means for receiving the amplified signal and converting the electrical signal to an acoustical signal.

Unite States atet [1 1 [111 3,898,382 Dalton, Jr. et al. Aug. 5, 1975 AUDIOMETRIC SIGNAL AND APPARATUS FOR PRODUCING SUCH SIGNAL [57] ABSTRACT Inventors: Leslie W. Dalton, Jr., P.O. Box

1162, Mesilla Park, N. Mex. 88047; James A. Boehm, III, PO. Box 4927, Las Cruces, N. Mex. 88003 Filed: Apr. 9, 1973 Appl. N0.: 349,104

Primary ExaminerKathleen H. Claffy Assistant ExaminerTommy P. Chin An audiometric signal for testing hearing is disclosed having alternating first and second portions, the first portion having a known frequency and a first amplitude, the second portion having the same known frequency and a second amplitude, the change of amplitude occurring in response to the zero crossing of a duty cycle base signal and coinciding with a zero crossing of the audiometric signal. In one exemplar embodiment, apparatus for generating the audiometric signal is provided, comprising means for generating a sinusoidal signal of preselected audio frequency, means for generating a duty cycle base signal of preselected frequency, gating means for receiving the duty cycle base and sinusoidal audio signals and alternately passing the sinusoidal audio signal of preselected frequency at first and second differing amplitudes, the change in amplitude occurring in response to the zero crossing of the duty cycle base signal and coinciding with a zero crossing of the sinusoidal audio signal, amplifying means for amplifying the signal, and means for receiving the amplified signal and converting the electrical signal to an acoustical signal.

11- Claims, 4 Drawing Figures G A B 0 3 A lPcPdlr are l 7 m fi s SQUARE DIV/DER LEVEL E: 20 I 30 WAVE J FILTERS GENERATOR fcmculrs CIRCUIT w t a EARPHONE 5 12 32 10 T 34 DIV/DER V CIRCUITS PATENTED AUG 5 I975 SHEET MZOIQQQM XOR Q UBWO mink mtqbGm KORQJQBWO m $3 mqim AUDIOMETRIC SIGNAL AND APPARATUS FOR PRODUCING SUCH SIGNAL BACKGROUND OF THE INVENTION This invention relates to an audiometric stimulus and apparatus for producing the stimulus for testing human hearing.

In the differential diagnosis of hearing disorders, two major physiological phenomenon and their interaction are measured. These are (l) recruitment of loudness, and (2) abnormal tone decay. All effort in instrument design has been directed toward determination of these two phenomena, with several procedures resulting that measure recruitment and decay.

Recruitment is a physiological by-product of a damaged cochlea (that part of the ear which converts mechanical energy into nerual energy), which compresses the absolute threshold (smallest sound audible to an ear) and the pain threshold close together. This compression does not occur at all frequencies of an individuals hearing range and therefore must be sought out using diagnostic procedures. In the past, two major techniques for indicating the presence of recruitment have remained in practice: (1) loudness balancing and (2) short increment sensitivity index (SISI). Loudness balancing requires a standard audiometer with two independent channels which allow the patient to compare either an unaffected ear to the diseased one or an unafected frequency to an abnormal one. The result can be plotted which reflects the compression earlier described. The SISl technique takes advantage of this increased sensitivity by presenting a series of l-dB tone bursts on top of a -dB above threshold continuous tone. A normal ear will not detect these bursts, but a diseased cochlea will pick up all or part with a percentage score reflecting how many.

Tone decay is characterized by the patients reporting a continuous unchanging tone as fading away when in fact it does not. This physiological problem is caused by the failure of the eighth cranial nerve to allow transmission because of some fatigue factor. A subjective judgement of the presence of tone decay can be made with a standard audiometer and a clock by simply timing the patients reports of when (or if) the unchanging tone appears to go away. A qualitative method of indicating eighth nerve involvement is through the use of a von Bekesy audiometer, as described in Pat. No. 2,563,384, which issued Aug. 7, 1951. The function of the von Bekesy audiometer is based on a classical psychophysical procedure of adjustment. This allows the patient to track his own threshold on an audiometer that is constantly changing frequency from low to high. In the differential diagnosis process, the patient is required to proceed through a specified frequency range twice with one sweep presenting pulsed tones (usually about 500 milliseconds off-on) and another presenting a continuous tone. If tone decay is present, then the continuous tracing will pull away" from the pulsed tracing, wholly or in part, thus indicating eighth nerve involvement. By contrast, a normal car will show very little, if any, variation in the two tracings.

All these procedures are, for the most part, specific in nature and require single use specialized apparatus for each test. Furthermore, each procedure must be run independent of the other, resulting in a rather long accumulation of time. In fact, time alone can cause fatigue in a patient which can influence testing. For example, the loudness balance procedure requires approximately 20 minutes; the von Bekesy procedure requires approximately 24 minutes; on top of this is the routine audiometric testing which must be added and which brings the total test time to approximately one hour. In addition, each test is carried out on a single purpose apparatus which requires a rather large investment of money and space. Furthermore, these procedures are threshold procedures, thus requiring sound proofed and sound retardant test environments.

Accordingly, one primary feature of the present invention is to provide a multi-component stimulus which allows for recruitment and tone decay testing with but a single audiometric signal.

Another feature of the present invention is that all testing is done at suprathreshold levels, thus eliminating the need for a sound retardant or sound proofed room.

Yet another feature of the present invention is that since only one piece of equipment is required, highly trained personnel are not necessary to operate the equipment.

Still another feature of the invention is that recruitment and tone decay testing can be accomplished in less than fifteen minutes with a single piece of apparatus.

SUMMARY OF THE INVENTION The present invention remedies the problems of the prior art by providing a single apparatus for generating a multicomponent audiometric signal which allows for recruitment and tone decay testing with but a single stimulus.

In accordance with a principle of this invention, an apparatus for generating an audiometric signal for testing hearing, comprising means for generating at least one sinusoidal signal of preselected audio frequency, means for generating a duty cycle base signal of preselected frequency, gating means for receiving said duty cycle base and sinusoidal audio signals and alternately passing said sinusoidal audio signals of preselected frequency at first and second preselected amplitudes, said change in amplitude occurring in response to the zero crossing of said duty cycle base signal and coinciding with a zero crossing of said sinusoidal audio signal, amplifying means for amplifying said signal, and means for receiving said amplified signal and converting said electrical signal to an acoustical signal is provided.

In accordance with a further principle of this invention, the apparatus for generating an audio signal for testing hearing comprises means for generating a plurality of sinusoidal signals of preselected audio frequencies, means for generating a duty cycle base signal of preselected frequency, and gating means for receiving said duty cycle base and selected one of said sinusoidal signals and alternately passing said sinusoidal signal at first and second preselected amplitudes, said change in amplitude occurring in response to the zero crossing of said duty cycle base signal and coinciding with a zero crossing of said selected sinusoidal audio signal.

In accordance with a further principle of this invention, an audiometric signal for testing hearing comprising alternating first and second portions, said first alternating portion including a known first frequency having a known first amplitude, said second alternating portion including said known first frequency and having a known second amplitude, said change of amplitude between said first and second alternating portions occurring in response to the zero crossing point of a duty cycle base signal and coinciding with a zero crossing point of said audiometric signal.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the manner in which the above-recited advantages and features of the invention are attained and can be understood in detail, a more particular description of the invention may be had by reference to specific embodiments thereof which are illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the appended drawings illustrate only typical embodiments of the invention therefore are not to be considered limiting of its scope for the invention may admit to further equally effective embodiments.

In the Drawings:

FIG. 1 is a block diagram schematic of one embodiment of the invention.

FIG. 2 is a wave form chart showing the relationship between the sinusoidal audiometric signal produced and the duty cycle base signal according to the invention.

FIG. 3 is a wave form diagram of the sign wave audiometric signal.

FIG. 4 is a block diagram schematic of a second embodiment of the apparatus according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 discloses one of the preferred embodiments of the invention. A square-wave generator generates a square-wave signal that is applied to divider circuits 12. Square-wave generator 10 may be a crystal oscillator generating a 1.0 MHz frequency square-wave, shown for example at A. The divider circuits 12 may be selectable cascaded divide-by-l6 counters that result in selectable square-waves having frequencies of 0.5 KHz, 1.0 KHz, 2.0 KHz, 4.0 KHz, and 8.1 KI-Iz as shown at B, these frequencies generally being those used in audiometric testing, namely 0.5, l, 2, 4, and 8 KI-Iz. The selected frequency divided square-wave is applied to a level circuit 14 which is an operational amplifier limiter circuit for furnishing a constant amplitude and spectral content wave form square-wave. The square-wave is then applied to a filter means 16 for converting the square-wave to a sine wave as shown at C. The filter may ideally be a four-pole Butterworth filter with 3-dB points at the five test frequencies. The sine wave as shown at C has the same period as the square-wave shown at B. The sine wave output of filter 16 is applied as an input to resistor R1 (18) and as an input to resistor R2 (24), each of which is in series with FETs Q1 and Q2, (20 and 22) respectively. The outputs of Q1 and Q2 are applied to a summing junction 23 at the input of operational amplifier 32. Operational amplifier 32 has a feedback resistor R3 (30) from the summing junction 23 to its output.

The output of the square-wave generator 10 is also applied to a divider circuit 26, the output of which is a 0.5 Hz squarewave duty cycle base signal, as shown at D, applied to the input of operational amplifier driver 28. The output of the operational amplifier driver 28 are two square-waves, l80out-of-phase, as shown at E and F, which are connected to the gate leads of FETs Q1 and Q2. The output of amplifier 32 is applied to earphone 34 for use by the patient undergoing hearing testing.

In operation, FETs Q1 and Q2 aregated on during the positive cycles of the out-of-phase squarewaves as shown at E and F that are applied to the gate leads of FETs Q1 and Q2. O1 is gated on during the positive one-half cycle of the duty cycle base squarewave shown at D while O2 is turned on during the negative one-half cycle of the duty cycle base square-wave shown at D. By adjusting the value of resistors R1 and R2, the gain of amplifier 32 may be varied so as to causea variation in the amplitude of the sine wave signals applied through Q1 and Q2. The gain of amplifier 32 when Q] is conducting is a function of R3/Rl while the gain when O2 is conducting is a function of R3/R2. The output of amplifier 32 is the composite signal shown at the G with the signal amplitude changing in response to the zero crossing points of sine wave C coinciding with the zero crossing of the duty cycle base signal shown at D.

Referring now to FIGS. 2 and 3, a basic diagram of the sine wave 42 stimulus is shown. The envelope 40 of the audiometric signal is shown, with the amplitude of the sine wave changing at zero crossing points 44. The sine wave signal changes in amplitude at X and Y as sine wave signal 42 makes a zero crossing as shown at 44 (see FIG. 2). The duty cycle base signal output from the divider circuit 26 (see FIG. 1) is shown at 46 and the zero crossing of the duty cycle base signal 46 is shown to coincide with the zero crossing 44 of the sine wave signal 42. At each transition of the duty cycle base signal 46 to the zero crossing, the sine wave signal 42 changes amplitude. In an actual patient test, the patients hearing threshold is indicated at I-IL, while the reference intensity of the audiometric signal 42 is shown at RI which is always maintained at 30 dB above the patients absolute theshold I-IL. The change of intensity A I indicates the amplitude differential between the reference intensity (RI) of the signal 42 and its increased amplitude intensity which occurs at X. At Y there is a duty cycle base signal transition and the sine wave signal 42 decreases in intensity from its increased amplitude to the reference intensity (RI) where it is maintained for the next half-cycle of the duty cycle base signal 46.

Accordingly, it may be seen that the audiometric signal for testing hearing comprises first and second portions. The first portion is defined by the interval or time period X-Y, which is a sine wave signal having a known first frequency and a known first amplitude, and the second portion is defined by the time interval Y-X in which the sine wave signal has the same known first frequency but has a second amplitude, the change in amplitude between the first and second alternating portions (X-Y, Y-X) coinciding at a zero crossing point 44 of thesine wave signal 42 and occurring in response to the zero crossing point of the duty cycle base signal 46. The first and second portions (X-Y, Y-X) alternate to provide a modulated pure tone changing in auditory level corresonding to the selected first frequency alternately changing in amplitude from the first to the second amplitudes.

In FIG. 3, a sample wave form of the audiometric signal at the output of power amplifier 32 is shown. The horizontal time scale and vertical amplitude have been adjusted to show the most detail at a ratio of 9.5 dB.

The sine wave signal of a first known frequency is shown at 42 and changes amplitude at the zero crossing point 44. The change in amplitude of the sine wave signal 42 is indicated by A l. Of course, when zero crossing of the duty cycle base signal and the sine wave signal are said to be coincident, we are not speaking of ideal coincidence but relative coincidence within design parameters of the filters and switching circuits. There will always be some delay response in the filters and same transient response in the FETs to prevent an ideal coincidence. Coincidence is used herein, then, means substantial coincidence within the design paramters of the circuit.

A second embodiment of the invention is shown in FIG. 4. A sine wave oscillator 50 is shown applying an output signal as shown at J, to the input of parallel resistors R4 and R5 (52 and 53) respectively. Resistor R4 is in series with FET Q3 (66) which is applied to a summing junction 67 as one input to operational amplifier 68. R5 (53) is also connected to summing junction 67 as one input to operational amplifier 68. A feedback resistor R6 (69) is applied across amplifier 68. The output of sine wave oscillator 50 is also applied as an input to a comparator circuit 54 which detects the zero crossings of the sine wave signals shown at J. The output of comparator 54 is a square-wave signal, shown at K, which is applied to a differentiating network comprising capacitor 55, resistor 56, and diode 57, which differentiates the positive going portions of the squarewave shown at K and applies these positive going pulses as one input to AND gates 58 and 59. A square-wave oscillator 62 generates a duty cycle base square-wave shown at M, and applies it as an input to AND gate 58. The output of square-wave oscillator 62 is also applied to inverter 60, the output of which is square-wave signal shown at L, 180 out-of-phase with the signal shown at M, which is applied as a second input to AND gate 59. The output of AND gate 58 is applied to the set (S) input of a flip-flop 64, while the output of AND gate 59 is applied to the reset (R) input of flip-flop 64. The output of flip-flop 64 is applied to the gate lead of PET Q3 (66).

In operation, when a positive going portion of the duty cycle base square-wave shown at M is present as an output from square-wave oscillator 62, one input to AND gate 58 will be at logic I. The logic output of the inverter 60 will be applied to one input of AND gate 59, thus disabling gate 59. As each pulse (shown at N) is applied to AND gate 58, a logic 1 input is applied to AND gate 58, thereby enabling the gate and applying a positive logic 1 to the S input of flip-flop 64. With a logic 1 applied to the S input of flip-flop 64, the output of flip-flop 64 is a logic I or a positive voltage applied to the gate lead of PET Q3 (66). O3 is thereby turned on and allows the sine wave signal from oscillator 50 to be applied via resistor R4 and FET 66 to the input of amplifier 68. Successive receipt of logic I pulses from AND gate 58 will not change the state of flip-flop 64. When the duty cycle base square-wave, shown at M, changes to its negative going half-cycle, AND gate 58 is disabled and AND gate 59 is enabled, applying a logic 1 signal to the R input of flip-flop 64, upon receipt of the next logic 1 signal from the differentiating network, capacitor 55, resistor 56 and diode 57. With the R input having a logic I, the output of flip flop 64 changes to a logic 0, therby gating off FET Q3 (66). With FET Q3 gated off, the sine wave output of oscillator 50 is applied via resistor R5 to summing junction 67 to the input of the amplifier 68. The output of flip-flop 64 is a square-wave function, shown at 0, which alternately gates FET 66 on and off during alternate cycles of the duty cycle square-wave M. AND gates 58 and 59 insure that the duty cycle sqrare-wave coincides with a zero crossing of the sine wave signal generatec by oscillator 50.

When FET O3 is conducting, the gain of amplifier 68 will be a function of R4, R5 and R6, while when 03 is not conducting the gain of amplifier 68 will only be a function of R5 and R6. The output of amplifier 68 is the wave form shown at P which is the audiometric signal shown in FIGS. 2 and 3 comprising a sine wave having a known first frequency and first and second amplitudes changing in coincidence with a zero crossing of the sine wave signal, and changing in amplitude in response to the zero crossing of the duty cycle squarewave produced by square-wave oscillator 62. The stimulus or audiometric signal is then applied to earphone 70 for patient testing.

It may be seen that the apparatus of FIG. 1 is, broadly speaking, an apparatus for generating an audiometric signal for testing hearing, comprising a square-wave generator for generating a square-wave signal of predetermined frequency, frequency dividing means for dividing the predetermined square-wave signal into a square-wave signal of preselected frequency, level adjusting means for adjusting the amplitude of the squarewave signal, filter means for converting the squarewave signals to sinusoidal signals corresponding in period to the square-wave signals, means for generating a duty cycle base square-wave signal of preselected period, gating means receiving the duty cycle base signal and the sinusoidal signal of preselected frequency and alternately passing the sinusoidal signal of preselected frequency at first and second preselected amplitudes, the change in amplitude occurring in response to each zero crossing of said duty cycle base signal and coinciding with the zero crossing of said sinusoidal signal, amplifying means for amplifying the signal, and an earphone for receiving the amplified signal and converting the signal to an acoustical signal.

In addition, the circuitry of FIGS. 1 and 4 may be summarized as apparatus for generating an audiometric signal for testing hearing, comprising means for generating at least one sinusoidal signal of preselected audio frequency, means for generating a duty cycle base signal of preselected'frequency and gating means for receiving the duty cycle base and sinusoidal audio signals and alternatelypassing said sinusoidal audio signal of preselected frequency at first and second preselected amplitudes, the change in amplitude occurring in response to each zero crossing of the duty cycle base signal and coinciding with a zero crossing of the sinusoidal audio signal.

In testing patients, the audiometric signal is applied to an earphone where the electrical signal is converted to an acoustical signal for application to the patientss ear at a level above threshold. In a normal car, a patient hearing the audiometric signal shown in FIG. 3 would detect only a modulated pure tone changing in auditory level corresponding to the sine wave signal 42 of a first frequency alternately changing in amplitude from a first to a second amplitude. A normal car would not detect any phenomenon occurring at the zero crossing points 44. When the A I of the stimulus is decreased to 2.5 dB or below, the normal ear will fail to distinguish a modulated tone and will note only a pure tone with no modulation.

In an ear having a diseased cochlea exhibiting recruitment, the diseased ear will hear a click" at the zero crossings 44 and will detect these clicks and the modulated pure tone to levels of A 1 below 2.5 dB, often times to levels such as 0.3 to 0.5 dB, before the modulated tone is not discerned. In an ear having a retrocochlear disease or lesion exhibiting tone decay, the

1 ear will hear auditory clicks occurring at zero crossing points 44 at levels above and below 2.5 dB, but the patientlose s the perception of the unchanging ongoing pure tone which appears to fade away within approximately ten seconds, thus leaving only the click as a discernible stimulus to the patient.

While measurements of 0.3 and 0.5 dB are difficult to attain, the absolute level of A I is not as important as the relative levels established by the apparatus. The ratio of the patients A I readings to normal standards is most important, and as long as the apparatus readings are consistent at the levels measured, the quantitative measurement levels are not critical. In all of the testing above, classical psychophysical methods of limits, ascending and descending series [a bracketing procedure] are utilized.

It may be noted of course that the testing frequencies selected were 0.5, l, 2, 4, and 8 KHZ, but these are frequencies selected as standard auditory testing frequencies. Other frequencies may possibly be utilized. The duty cycle base signal that was utilized was 0.5 Hz, but of course, other frequencies may be utilized to achieve the same results.

Numerous variations and modifications may obviously be made in the structure herein described without departing from the present invention. Accordingly, it should be clearly understood that the forms of the invention herein described and shown in the figures of the accompanying drawings are illustrative only and are not intended to limit the scope of the invention.

What is claimed is:

1. An audiometric signal for testing an ear for hearing comprising alternate first and second portions having a known frequency, said first portion having a known first constant amplitude and said second portion having a known different second constant amplitude, said change of amplitude between said alternate signal portions coinciding with a zero crossing point of said audiometric signal.

2. The audiometric signal for testing an ear for hearing described in claim 1, wherein the period of said alternate signal portions is controlled by a duty cycle base signal of preselected frequency, said change of amplitude between said alternate signal portions occurring in response to the zero crossing point of said duty cycle base signal.

3. The audiometric signal for testing an ear for hearing described in claim 1, wherein said first and second portions continuously alternate to provide a continuous audiometric signal for which the frequency is fixed at said known frequency.

4. The audiometric signal for testing an ear for hearing described in claim 3, wherein said change of amplitude between said first and second portions occurs in response to the zero crossing point of a duty cycle base signal for controlling the period of said first and second portions.

5. An audiometric signal for testing an ear for hearing comprising alternate first and second portions having a known frequency, said first portion having a known first constant amplitude and said second portion having a known different second costant amplitude, said change of amplitude between said alternate signal portions coinciding with a zero crossing point of said signal, the period of said alternate signal portions being contolled by a duty cycle base signal of preselected frequency, said change of amplitude between said alternate signal portionsoccurring in response to the zero crossing point of said duty cycle base signal.

6. Apparatus for generating an audiometric signal for testing an ear for hearing comprising square-wave generation means for generating a squarewave signal of predetermined frequency, level adjusting means for adjusting the amplitude of said square-wave signal,

filter means for converting said square-wave signal to a sinusoidal signal corresponding in period to said square-wave signal,

means for generating a duty cycle base square-wave signal of preselected period,

gating-means receiving said duty cycle base signal and said sinusoidal signal of preselected frequency and alternately passing said sinusoidal signal of preselected frequency at first and second preselected different constant amplitudes, said change in amplitude occurring in response to each zero crossing of said duty cycle base signal and coinciding with a zero crossing of said sinusoidal signal, amplifying means for. amplifying said signal, and

an earphone for receiving said amplified signal and converting said signal to an acoustical signal.

7. Apparatus for generating an audiometric signal for testing hearing, comprising means for generating at least one sinusoidal signal of preselected audio frequency,

means for generating a duty cycle base signal of preselected frequency,

gating means for receiving said duty cycle base and sinusoidal audio signals and alternately passing said sinusoidal audio signal of preselected frequency at first and second preselected different constant amplitudes, said change in amplitude occurring in response to the zero crossing of said duty cycle base signal and coinciding with a zero crossing of said sinusoidal audio signal,

amplifying means for amplifying said signal, and

means for receiving said amplified signal and converting said electrical signal to an acoustical signal.

8. Apparatus for generating an audiometric signal for testing hearing, comprising means for generating a plurality of sinusoidal signals of preselected audio frequencies,

means for generating a duty cycle base signal of preselected frequency, and

gating means for receiving said duty cycle base and a selected one of said sinusoidal audio signals and alternately passing said selected sinusoidal audio signal at first and second preselected different constant amplitudes, said change in amplitude occurring in response to the zero crossing of said duty cycle base signal and coinciding with a zero crossing of said selected sinusoidal audio signal.

9. The apparatus as described in claim 8, including detecting for the presence of modulated pure tone and of audible clicks at differences in amplitude levels between said first and second portions below 2.5 dB as an indication of recruitment, and detecting for the presence of audible clicks at different frequencies and amplitude levels between said first and second portions above and below 2.5 dB and for the fading loss of pure tone as an indication of tone decay. 11. The method as described in claim 10 wherein said testing is performed at about 30 dB above the patients absolute threshold. 

1. An audiometric signal for testing an ear for hearing comprising alternate first and second portions having a known frequency, said first portion having a known first constant amplitude and said second portion having a known different second constant amplitude, said change of amplitude between said alternate signal portions coinciding with a zero crossing point of said audiometric signal.
 2. The audiometric signal for testing an ear for hearing described in claim 1, wherein the period of said alternate signal portions is controlled by a duty cycle base signal of preselected frequency, said change of amplitude between said alternate signal portions occurring in response to the zero crossing point of said duty cycle base signal.
 3. The audiometric signal for testing an ear for hearing described in claim 1, wherein said first and second portions continuously alternate to provide a continuous audiometric signal for which the frequency is fixed at said known frequency.
 4. The audiometric signal for testing an ear for hearing described in claim 3, wherein said change of amplitude between said first and second portions occurs in response to the zero crossing point of a duty cycle base signal for controlling the period of said first and second portions.
 5. An audiometric signal for testing an ear for hearing comprising alternate first and second portions having a known frequency, said first portion having a known first constant amplitude and said second portion having a known different second costant amplitude, said change of amplitude between said alternate signal portions coinciding with a zero crossing point of said signal, the period of said alternate signal portions being contolled by a duty cycle base signal of preselected frequency, said change of amplitude between said alternate signal portions occurring in response to the zero crossing point of said duty cycle base signal.
 6. Apparatus for generating an audiometric signal for testing an ear for hearing comprising square-wave generation means for generating a squarewave signal of predetermined frequency, level adjusting means for adjusting the amplitude of said square-wave signal, filter means for converting said square-wave signal to a sinusoidal signal corresponding in period to said square-wave signal, means for generating a duty cycle base square-wave signal of preselected period, gating means receiving said duty cycle base signal and said sinusoidal signal of preselected frequency and alternately passing said sinusoidal signal of preselected frequency at first and second preselected different constant amplitudes, said change in amplitude occurring in response to each zero crossing of said duty cycle base signal and coinciding with a zero crossing of said sinusoidal signal, amplifying means for amplifying said signal, and an eArphone for receiving said amplified signal and converting said signal to an acoustical signal.
 7. Apparatus for generating an audiometric signal for testing hearing, comprising means for generating at least one sinusoidal signal of preselected audio frequency, means for generating a duty cycle base signal of preselected frequency, gating means for receiving said duty cycle base and sinusoidal audio signals and alternately passing said sinusoidal audio signal of preselected frequency at first and second preselected different constant amplitudes, said change in amplitude occurring in response to the zero crossing of said duty cycle base signal and coinciding with a zero crossing of said sinusoidal audio signal, amplifying means for amplifying said signal, and means for receiving said amplified signal and converting said electrical signal to an acoustical signal.
 8. Apparatus for generating an audiometric signal for testing hearing, comprising means for generating a plurality of sinusoidal signals of preselected audio frequencies, means for generating a duty cycle base signal of preselected frequency, and gating means for receiving said duty cycle base and a selected one of said sinusoidal audio signals and alternately passing said selected sinusoidal audio signal at first and second preselected different constant amplitudes, said change in amplitude occurring in response to the zero crossing of said duty cycle base signal and coinciding with a zero crossing of said selected sinusoidal audio signal.
 9. The apparatus as described in claim 8, including amplifying means for amplifying said signal from said gating means, and means for translating said electrical signal output of said amplifier into an acoustical signal.
 10. Method of audiometrically testing an ear simultaneously for recruitment and tone decay, comprising applying a sinusoidal audio signal characterized by a single frequency within the audio range continuously alternating between a first portion at a first constant amplitude and a second portion at a different second constant amplitude, the change therebetween occurring at a duty cycle rate at zero crossing points, detecting for the presence of modulated pure tone and of audible clicks at differences in amplitude levels between said first and second portions below 2.5 dB as an indication of recruitment, and detecting for the presence of audible clicks at different frequencies and amplitude levels between said first and second portions above and below 2.5 dB and for the fading loss of pure tone as an indication of tone decay.
 11. The method as described in claim 10 wherein said testing is performed at about 30 dB above the patient''s absolute threshold. 